If it matters to you, Consumer Reports ran a big test on multi-functions this month (May issue), and Canon had 3 models in the top 5 for over-all value. They did not test the MP500, just the MP450. But the MP500 is easily your best choice IMHO. There's just no substitute for individual ink tanks.
Here's the ratings in order:
I think the MP500 is very similar to the MP800